1. Field of the Invention
The present invention relates to a power supply circuit for use in an electronic apparatus such as a video projector or a television receiver.
2. Description of the Prior Art
Recently in an electronic apparatus such as a video projector, a television receiver and so forth for projecting image, there is employed a system which, even when a main power source thereof is placed in its off-state, enables a user to close the main power source again by turning on a main power switch of the apparatus by means of a remote controller or the like.
The power supply circuit of this type known heretofore has such a circuit configuration as that shown in FIG. 3. The power supply circuit shown in FIG. 3 has a power plug 1000, a primary circuit 1001 and a secondary circuit 1002. The power plug 1000 is capable of inserting into, e.g., an outlet of a commercial AC power source for supplying an AC input voltage. The power plug 1000 is connected to a relay switch 1003, a rectifier diode bridge 1004 and so forth in the primary circuit 1001 of transformers 1020 and 1021. The rectifier diode bridge 1004 is connected to a main switching power circuit 1005. Power is supplied to a standby switching power circuit 1006 from a half-wave rectifier circuit consisting of a diode 1031 and a capacitor 1032.
The secondary circuit 1002 of the transformers 1020 and 1021 is equipped with a relay controller 1007 and a microcomputer 1009 for turning on or off the relay switch 1003. The relay switch 1003 is disposed in the primary circuit 1001 relative to the main switching power circuit 1005 and the standby switching power circuit 1006. Meanwhile a relay control coil 1008 is disposed in the secondary circuit 1002 relative to the main switching power circuit 1005 and the standby switching power circuit 1006.
In the known power supply circuit of FIG. 3 mentioned above, it is necessary to dispose the secondary coil, the rectifier and the regulator of the standby switching power circuit 1006 on the side of the secondary circuit 1002 side, and also to dispose the relay control coil 1008 and the power switch 1010 on the same side for turning on the main switching power circuit 1005, and further to dispose the relay switch 1003 and the main switching power circuit 1005 in the primary circuit 1001. When the power switch 1010 is turned on in a state where an on-control signal from the microcomputer 1009 is applied to the base of a transistor 1030, then the transistor 1030 is switched on to cause a current flow in the relay control coil 1008, thereby turning on the relay switch 1003. Consequently, the main switching power circuit 1005 is driven to deliver a desired main power output 1011.
Since the relay switch 1003 and the relay control coil 1008 are housed normally in one casing, the secondary circuit 1002 including the relay control coil 1008 is so positioned as to be proximate to the primary circuit 1001 with partial entry.
However, in view of conforming with required safety standards, a certain electrical insulation distance needs to be insured between the primary circuit 1001 and the secondary circuit 1002.
For this reason, it is necessary to space the primary circuit structure and the secondary circuit structure apart from each other by increasing the area of a substrate where the primary circuit 1001 including the relay switch 1003 and the secondary circuit 1002 including the relay controller 1007 are formed.
Similarly in view of conforming with the required safety standards, the level of any spurious radiation noise generated in the electronic apparatus and leaking via its power cord needs to be reduced under a predetermined value. In preventing harmful influence of such spurious radiation noise generated from the secondary circuit 1002 and induced to the primary circuit 1001, there exists a problem of necessitating increase of the substrate area to reduce the leakage level of the spurious radiation noise from the secondary circuit 1002 to the primary circuit 1001.